Role of Borderline Solvents to Induce Pronounced Extended-Chain Lamellar Order in π-Stackable Polymers

Abstract

Thin films of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (C14–PBTTT) exhibit a monolayer-terraced morphology that indicates a pronounced lamellar order with π-stacks of extended polymer chains. Previously this remarkable state of order was thought to be promoted by the interdigitation of alkyl side chains between the lamellae during cooling from the liquid-crystalline (LC) phase. Here we establish that the key to this ordering in fact is the formation of unentangled π-stacks of extended polymer chains in dilute solutions of chlorobenzene (CB) or 1,2-dichlorobenzene (o-DCB), which though routinely used as the “best” solvents are in fact borderline solvents. Film formation causes these π-stacks to deposit substantially oriented in the film plane, while the subsequent anneal and cool from LC phase accentuates this incipient order to develop the monolayer-terraced morphology. This mechanism is supported by the following lines of evidence. (i) Hydrodynamic and viscometry measurements respectively of the Kuhn segment length and Mark–Houwink–Sakurada exponent of PBTTT reveal that CB is a near-Θ solvent, and PBTTT is significantly stiffer than regioregular polythiophene. (ii) Solution-state UV–vis spectroscopy reveals an early coil → rod transition in highly dilute solutions, which gives rise to unentangled π-stacks. (iii) Solid-state UV–vis spectroscopy, atomic force microscopy and variable-angle spectroscopic ellipsometry together reveal the as-deposited π-stacks are already substantially oriented in the film plane. We further demonstrate that this monolayer-terraced morphology can also be induced in regioregular poly(3-hexylthiophene) films using a borderline solvent mixture of chlorobenzene and mesitylene, and in very dilute CB where the incipient π-stacks do not entangle. Therefore, this dilute π-stacking mechanism is general. Processing with a borderline solvent or solvent additive thus provides a general route to obtain superior supramolecular order in π-stackable conjugated polymers.

Structure and Morphology Control in Thin Films of Conjugated Polymers for an Improved Charge Transport

“…Utilizing single molecule spectroscopy (SMS), Hu et al also reported that with the gradually more intense conjugation in chains brought by a better planarity (for PQT12, P3), highly delocalized thienothiophene fused aromatic rings (for pBTTT) and a much higher planarity due to intra-chain donor-acceptor charge transfer (for pTzQT, P8) compared than P3HT, the content of single chains with an extended conformation in solutions increased, which suggested a more ordered and aligned rod-like film morphology arising from these rod-like single chains [109]. Thus, it is proven that the conformations of conjugated polymers display more anisotropic and extended characteristics as a result of planarity and rigidity [68,69,109], which have an important effect on the self-assembly and film morphology in terms of the stacking order.…”

“…It is known that a polymer chain could be regarded as its equivalent Kuhn chains, in which the motion units are modeled as Kuhn segments made of several repeating units [68,69]. Due to the large bending barrier of stiff conjugated polymers, the Gaussian chain model fitting well with flexible chains [Equation (2)] is replaced by worm-like chain model [Equation (3)] to better depict the conformation of rigid conjugated polymers,…”

“…Firstly, the elongated persistence length of chains of conjugated polymers makes their single chains display a more anisotropic and extended conformation with less bending like paperclips, compared to the isotropic and disordered coils of more flexible chains ( Figure 5a) [69]. Zhao et al proved that the persistence length of pBTTT (9.0 nm) was much larger than rr-P3HT (4.8 nm) due to the introduction of fused thieno[3,2-b]thiophene moiety into backbones to strengthen the conjugation, implying more rigid chains than flexible chains like PS (2.8 nm) [68]. These conjugated polymer chains display more rod-like conformations, which tend to crystallize driven by intense π-π interaction of conjugation planes different from flexible chains.…”

“…[107] Besides the electronic effects, the different conjugated structures of backbones could lead to big changes of rigidity among the family of conjugated polymers, which influence the structures and morphology in turn. The rigidity of chains could be increased by a more intense conjugation [68,79,109]. Zhao et al reported that the persistence length of pBTTT (P4) (9.0 nm) was much larger than rr-P3HT (4.8 nm) due to the introduction of fused thieno [3,2-b]thiophene moiety into backbones to strengthen the conjugation, implying a more rigid chains [68].…”

“…The rigidity of chains could be increased by a more intense conjugation [68,79,109]. Zhao et al reported that the persistence length of pBTTT (P4) (9.0 nm) was much larger than rr-P3HT (4.8 nm) due to the introduction of fused thieno [3,2-b]thiophene moiety into backbones to strengthen the conjugation, implying a more rigid chains [68]. More rigid chains tend to adopt extended rod-like conformation, which is prone to aggregate orderly and form the precursor for domains with a large size and good performance in pBTTT films.…”